[Elecraft] ANTENNAS-ANTENNAS-ANTENNAS

[k6se@juno.com]

K2MUF wrote:

"I have two Cushcraft Model PC804N, 806 to 902mhz 4 element yagi
antenna's.  I will be mounting them for vertical reception, one under the
other on the same mast. I will be using a type "N" tee to couple the two
antenna's to the feed cable.

My questions are:
1:    What is the optimum spacing required between antenna's. ( the tips
of the longest elements )"
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Optimnum spacing is 0.7 wavelengths, boom-to-boom.  At 850 MHz, that's
about 9.75 inches.
==========

"2:    If each antenna has a gain of 4db what type of gain can I expect
from combining two of them this way. Will it double to 8db??"
----------
Doubling the power results in 3 dB gain.  With 0.7 wavelength spacing you
can expect 3 dB gain.  If each Yagi has 4 dBd gain (I would expect that a
4-el Yagi has 2 or 3 dB gain more than that), you'll have a total of 7
dBd gain.  Just add 3 dB to the gain of a single Yagi.
==========

"3: What is the impedance of RG-6 cable."
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The characteristic impedance of RG-6 coax is 75.0 ohms.
==========

"It is recommended for use as the transmission line for a 300 ohm
commercial wide band VHF UHF antenna with a ballon using an "F"
connector.  Is this RG-6 plain TV antenna coax you buy at Radio
Shack.???"
----------
RG-6 is low-loss (relatively) with a foam dielectric and was commonly
used in C-band satellite TV systems for bringing the 950 to 1450 MHz
signal from the downconverter located at the dish antenna to the receiver
in the house.  I believe that RG-6 is slightly larger in diameter than
RG-59 (which is what Radio Shack sells).  F-connectors which will
accommodate RG-6 can be bought, if you puchase RG-6.

If you buy RG-59 from Radio Shack, try to get the foam dielectric type
because it has lower loss than the type that has polyethylene dielectric
(coax losses are considerably higher at UHF than at HF or VHF, so
limiting that loss is important).

The lengths of the feedlines from each output of the tee to the baluns at
the feedpoints of both Yagis must be equal to ensure the Yagis are fed in
phase.  However, you can't just combine two feedlines at the tee to one
feedline to the radio unless you use some method of matching impedances,
i.e., combining two 75-ohm coaxes from the Yagis would make the 75-ohm
feedline to the radio "think" it's looking into a 37.5-ohm load which
would result in a 2:1 SWR on the line.  This is intolerable at UHF.  If
50-ohm coax was run from the tee to the radio, the SWR on that line would
be 1.33:1.

I suggest that you consult the ARRL Anrtenna Book for information on
stacking antennas and methods of matching.

73, de Earl, K6SE